The present invention relates to a high frequency apparatus having an adjusting element.
A conventional high frequency apparatus will be explained hereinafter. As shown in FIG. 8, the conventional high frequency apparatus has been constructed to comprise an input terminal 1 to which a high frequency signal is input, a bandpass filter 2 connected to the input terminal 1, an unbalance-balance converter 3 connected to an output of the bandpass filter 2, a balanced mixer 6 one input of which an output of the unbalance/balance converter 3 is connected to and other input of which an output of a voltage-controlled oscillator 4 is connected to via a buffer 5, an intermediate frequency filter 7 connected between both outputs of the balanced mixer 6, a balance-unbalance converter 8 connected to the intermediate frequency filter 7, an output terminal 9 connected to an output of the balance-unbalance converter 8, a known PLL circuit 10 connected to the voltage-controlled oscillator 4, for controlling an oscillation frequency of the voltage-controlled oscillator 4, and a data input terminal 11 for supplying local oscillation frequency designating data to the PLL circuit 10. In addition, in order to adjust this high frequency device, test terminals 12, 13 are provided to both ends of the intermediate frequency filter 7 respectively.
An operation of the high frequency apparatus constructed as above will be explained hereinbelow. A high frequency signal input from the input terminal 1 is passed through the bandpass filter 2. The high frequency signal passed through the bandpass filter 2 is mixed with an output signal of the voltage-controlled oscillator 4, which is decided by the PLL circuit 10 according to a signal input from the data input terminal 11, by the balanced mixer 6 and then a signal having a sum or difference frequency is output from the balanced mixer 6. In other words, the high frequency signal input from the input terminal 1 can be tuned based on a data signal input from the data input terminal 11. A signal having a frequency bandwidth of 54.25 MHz to 58.75 MHz can be selected by passing the output of the balanced mixer 6 through the intermediate frequency filter 7, then converted into an unbalanced output by the succeeding balance-unbalance converter 8, and then output from the output terminal 9.
The bandpass filter 2 is a high frequency filter which consists of an inductance and a capacitance and has a passband characteristic shown as a characteristic 41 in FIG. 9A. The high frequency signal is waveform-shaped to have such passband characteristic 41. The intermediate frequency filter 7 is composed of a parallel circuit of an inductance 14 and a capacitance 15 and has a passband characteristic shown as a characteristic 42 in FIG. 9B. The intermediate frequency signal is waveform-shaped to have such passband characteristic 42. However, since the bandpass filter 2 and the intermediate frequency filter 7 are connected in series with each other, an output waveform output from the output terminal 9 can be given by a synthesized waveform shown as a characteristic 43 in FIG. 3C. Because of an influence of the intermediate frequency filter 7, it has been difficult to adjust the bandpass filter 2 while monitoring such synthesized waveform 43, i.e., obtain the characteristic 41 shown in FIG. 9A. Therefore, after both ends 12, 13 of the intermediate frequency filter 7 have been short-circuited and dumped by a contactor pin 17 having a dumping resistor 16 so as to eliminate the influence of the intermediate frequency filter 7, the bandpass filter 2 has been adjusted while monitoring the output waveform output from the output terminal 9.
However, according to the above configuration, the influence of the intermediate frequency filter 7 has not been able to be completely eliminated because of variations of a length of the contactor pin 17 and the dumping resistor 16, a contact resistance of the contactor pin 17, or the like. As a consequence, an adjusting error of the bandpass filter 2 serving as the high frequency filter has been caused.